Cache memory device and non-transitory computer readable recording medium

ABSTRACT

According to one embodiment, a cache memory device includes a nonvolatile cache memory, write unit, determination unit, selection unit, and erase unit. The nonvolatile cache memory includes a plurality of erase unit areas. Each of the erase unit areas includes a plurality of write unit areas. The write unit writes data to the nonvolatile cache memory. The determination unit determines whether the plurality of erase unit areas satisfy an erase condition or not. The selection unit selects an area to be erased from the plurality of erase unit areas when the plurality of erase unit areas satisfy the erase condition. The erase unit erases the data written to the area to be erased.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from U.S. Provisional Application No. 62/097,530, filed Dec. 29, 2014; and Japanese Patent Application No. 2015-038997, filed Feb. 27, 2015, the entire contents of all of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a cache memory device and a non-transitory computer readable recording medium.

BACKGROUND

A solid state drive (SSD) includes a nonvolatile semiconductor memory such as a NAND flash memory. The NAND flash memory includes a plurality of blocks (physical blocks). The plurality of blocks include a plurality of memory cells arranged at crossing points of word lines and bit lines.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a block diagram showing a configuration example of an information processing device including a cache memory device according to a first embodiment;

FIG. 2 is a flowchart showing an example of first cache control of the first embodiment;

FIG. 3 is a flowchart showing an example of second cache control of the first embodiment;

FIG. 4 is a flowchart showing an example of third cache control of the first embodiment;

FIG. 5 is a flowchart showing an example of fourth cache control of the first embodiment;

FIG. 6 is a block diagram showing a configuration example of an information processing system according to a second embodiment;

FIG. 7 is a flow chart showing an example of a process performed by the information processing system according to the second embodiment;

FIG. 8 is a block diagram showing an example of a detail configuration of an information processing system according to a third embodiment; and

FIG. 9 is a perspective view showing an example of a storage system according to the third embodiment.

DETAILED DESCRIPTION

In general, according to one embodiment, a cache memory device includes a nonvolatile cache memory, write unit, determination unit, selection unit, and erase unit. The nonvolatile cache memory includes a plurality of erase unit areas. Each of the erase unit areas includes a plurality of write unit areas. The write unit writes data to the nonvolatile cache memory. The determination unit determines whether the plurality of erase unit areas satisfy an erase condition or not. The selection unit selects an area to be erased from the plurality of erase unit areas when the plurality of erase unit areas satisfy the erase condition. The erase unit erases the data written to the area to be erased.

Embodiments will be described hereinafter with reference to drawings. In a following description, the same reference numerals denote components having nearly the same functions and arrangements, and a repetitive description thereof will be given if necessary.

First Embodiment

A cache memory device including a nonvolatile cache memory is described in the present embodiment. In the present embodiment, data is collectively erased per erase unit area in the nonvolatile cache memory. The erase unit area includes a plurality of write unit areas and a plurality of read unit areas.

In the present embodiment, a NAND flash memory is used as a nonvolatile cache memory and a nonvolatile memory. However, each of the nonvolatile cache memory and the nonvolatile memory may be a memory other than the NAND flash memory when the memory satisfies the above relationship among the erase unit area, the write unit area and the read unit area.

When the nonvolatile cache memory and the nonvolatile memory are the NAND flash memories, the erase unit area corresponds to a block. The write unit area and the read unit area correspond to a page.

In the present embodiment, for example, the erase unit area may be controlled in the other unit of, for example, two blocks, which allows data to be collectively erased.

In the present embodiment, access indicates both writing data to a memory device and reading data from the memory device.

FIG. 1 is a block diagram showing a configuration example of an information processing device including a cache memory device according to the present embodiment.

An information processing system 35 includes an information processing device 17 and an SSD 5. The information processing device 17 may be a host device corresponding to the SSD 5.

The information processing device 17 includes a processor 2, a memory 3 and a nonvolatile cache memory 4. The SSD 5 may be included in the information processing device 17 or may be connected to the information processing device 17 so as to transmit and receive data via the network, etc. Instead of the SSD 5, the other nonvolatile memory device such as a hard disk drive (HDD) may be used.

The information processing device 17 includes a cache memory device including a cache control unit 9, the memory 3 storing management information 61 to 64 and the nonvolatile cache memory 4. However, all or a part of the cache control unit 9, the management information 61 to 64, the memory 3 and the nonvolatile cache memory 4 may be provided outside the information processing device 17.

The nonvolatile cache memory 4 includes block groups BG₁ to BG₄. The nonvolatile cache memory 4 has an access speed higher than that of the SSD 5. The block group (first group) BG₁ includes blocks (first erase unit areas) B_(1,1) to B_(1,K). The block group BG₁ stores data accessed by the processor 2 (i.e., data used by the processor 2).

In the present embodiment, when the block group BG₁ satisfies an erase condition (first erase condition), a block to be erased (block to be discarded or pushed out) (first area to be erased) is selected from the blocks B_(1,1) to B_(1,K) in the block group BG₁ based on first-in first-out (FIFO).

For example, the erase condition is satisfied when data amount of each of the blocks B_(1,1) to B_(1,K) of the block group BG₁ exceeds a predetermined value. For example, the erase condition may be satisfied when the number of pages written to each of the blocks B_(1,1) to B_(1,K) of the block group BG₁ exceeds a predetermined number.

Data written to the block to be erased selected from the blocks B_(1,1) to B_(1,K) based on FIFO is written to a block group BG₂ when the data is in a first low-use state (for example, when the data is accessed for less than a set first number of times or at less than a set first frequency). In contrast, the data written to the block to be erased selected from the blocks B_(1,1) to B_(1,K) is written to a block group BG₃ when the data is in a first high-use state (for example, when the data is accessed for the first number of times or more or at the first frequency or more). The data written to the block to be erased selected from the blocks B_(1,1) to B_(1,K) is erased (i.e., discarded or pushed out) per block.

The block group (second group) BG₂ includes blocks (second erase unit areas) B_(2,1) to B_(2,L). The block group BG₂ stores the data in the first low-use state of the data written to the block to be erased selected from the block group BG₁.

In the present embodiment, when the block group BG₂ satisfies an erase condition (third erase condition), a block to be erased (third area to be erased) is selected from the blocks B_(2,1) to B_(2,L) in the block group BG₂ based on FIFO.

Data written to the block to be erased selected from the blocks B_(2,1) to B_(2,L) by FIFO is erased when the data is in a third low-use state (for example, when the data is accessed for less than a set third number of times or at less than a set third frequency). In contrast, the data written to the block to be erased selected from the blocks B_(2,1) to B_(2,L) is written to a block group BG₃ when the data is in a third high-use state (for example, when the data is accessed for the third number of times or more or at the third frequency or more). Then, the data written to the block to be erased selected from the blocks B_(2,1) to B_(2,L) is erased per block.

The block group (third group) BG₃ includes blocks (third erase unit areas) B_(3,1) to B_(3,M). The block group BG₃ stores the data in the first low-use state of the data written to the block to be erased selected from the block group BG₁. The block group BG₃ also stores the data in the third high-use state of the data written to the block to be erased selected from the block group BG₂.

In the present embodiment, when the block group BG₃ satisfies an erase condition (second erase condition), a block to be erased (second area to be erased) is selected from the blocks B_(3,1) to B_(3,M) in the block group BG₃ based on FIFO.

Data written to the block to be erased selected from the blocks B_(3,1) to B_(3,M) by FIFO is written to the block group BG₄ when the data is in a second low-use state (for example, when the data is accessed for less than a set second number of times or at less than a set second frequency). In contrast, the data written to the block to be erased selected from the blocks B_(3,1) to B_(3,M) is written to the other block in the block group BG₃ again when the data is in a second high-use state (for example, when the data is accessed for the second number of times or more or at the second frequency or more). Then, the data written to the block to be erased selected from the blocks B_(3,1) to B_(3,M) is erased per block.

The block group (fourth group) BG₄ includes blocks (fourth erase unit areas) B_(4,1) to B_(4,N). The block group BG₄ stores the data in the second low-use state of the data written to the block to be erased selected from the block group BG₃.

In the present embodiment, when the block group BG₄ satisfies an erase condition (fourth erase condition), a block to be erased (fourth area to be erased) is selected from the blocks B_(4,1) to B_(4,N) in the block group BG₄ based on FIFO.

Data written to the block to be erased selected from the blocks B_(4,1) to B_(4,N) by FIFO is erased.

In the present embodiment, FIFO is used as a method for selecting a block to be erased from each of the block groups BG₁ to BG₄. By selecting the block to be erased by FIFO, erasure is sequentially performed in each of the block groups BG₁ to BG₄ from a block having the oldest write time and write order. However, the block to be erased may be selected, for example, at random, or based on least recently used (LRU) or least frequently used (LFU). For example, the management information 61 to 64 includes identification information of the data, information indicating whether the data is data to be deleted or not, and state-of-use information of the data. A block having the largest amound of invalid data or a block having an amount of invalid data larger than a predetermined amount may be selected as the block to be erased based on the management information 61 to 64. For example, a block having the largest amount of invalid data and data to be deleted(deletion target data) or a block having an amount of invalid data and data to be deleted larger than a predetermined amount may be selected as the block to be erased based on the management information 61 to 64.

The memory 3 stores various types of control data such as the management information (list) 61 to 64 and address translation information 7. The memory 3 may be a volatile memory such as a dynamic random access memory (DRAM) or a static random access memory (SRAM) or may be a nonvolatile memory. The memory 3 may be included in the nonvolatile cache memory 4.

In the present embodiment, the cache control unit 9 can recognize identification information (for example, a logical address provided from the host (for example, Logical Block Addressing)) of cached data, a position to which the data is written and a state of use of the data based on the management information 61 to 64 and the address translation information 7. For example, the cache control unit 9 can select data cached to each of the block groups 5G; to BG₄ and a block erased by FIFO based on the management information 61 to 64 and the address translation information 7.

The management information 61 to 64 is metadata for the data written to the block groups BG₁ to BG₄, respectively. For example, the management information 61 to 64 includes information indicative of the state of use of the respective data by the processor 2. For example, the management information 61 to 64 includes identification information of the respective data, deletion information indicative of whether the data is data to be deleted or not, valid/invalid information indicative of whether the data is valid data or not, and cache determination information to determine whether the erase condition for erasing the block is satisfied.

The deletion information is information indicating that a delete command of the data is issued. More specifically, the deletion information is information, etc., indicating that a delete command of the data is received from an application program or an operating system (OS) executed by the processor 2. In the present embodiment, the deletion information includes, for example, information relating the identification information of each block to a logical address indicative of data to be deleted written to each block.

The valid/invalid information is information indicating that, for example, when the same data is written to a plurality of positions, the latest data is valid data and data other than the latest data is invalid data. In other words, for example, the valid data is updated data in the case where the update of the data written to the nonvolatile cache memory 4 is performed. For example, the invalid data is data which is not updated in the case where the update is performed. In the present embodiment, the valid/invalid information includes, for example, information relating the identification information of each block to a logical address indicative of valid data or invalid data written to each block.

The cache determination information is information, etc., including, for example, at least one of write information and read information per data, or at least one of write information and read information per block.

The write information includes, for example, at least one of write times, write numbers, write frequencies and write orders.

The read information includes, for example, at lest one of read times, read numbers, read frequencies and read orders.

For example, the address translation information 7 relates a logical address of data to a physical address of the nonvolatile cache memory 4 corresponding to the logical address (for example, Physical Block Addressing). The address translation information 7 is managed, for example, in a table form.

The processor 2 functions as an address translation unit 8 and a cache control unit 9 by executing a program stored in a memory of the processor 2, the memory 3, the nonvolatile cache memory 4 or the SSD 5.

In the present embodiment, the program to cause the processor 2 to function as the address translation unit 8 and the cache control unit 9 may be, for example, the OS, middleware or firmware. In the present embodiment, all or a part of the address translation unit 8 or all or a part of the cache control unit 9 may be implemented by hardware.

The address translation unit 8 generates information relating a logical address of write data to a physical address indicative of a position in the nonvolatile cache memory 4 in which the write data is stored, and registers the generated information to the address translation information 7.

When a logical address of read data is received from the processor 2, the address translation unit 8 translates the logical address to the physical address based on the address translation information 7.

The cache control unit 9 executes cache control for the nonvolatile cache memory 4 having the access speed higher than that of the SSD 5. For example, the cache control unit 9 manages data and logical and physical addresses indicative of the data by a write through method or a write back method.

In the write through method, data is stored in the nonvolatile cache memory 4 and also in the SSD 5.

In the write back method, data stored in the nonvolatile cache memory 4 is not stored in the SSD 5 together. The data is first stored in the nonvolatile cache memory 4, and then data pushed out from the nonvolatile cache memory 4 is stored in the SSD 5.

The cache control unit 9 includes a generation unit 10, control units 11 to 14 and variation units 15 and 16.

The generation unit 10 generates management information 61 to 64 corresponding to the block groups BG₁ to BG₄, and writes the management information 61 to 64 to the memory 3.

The control units 11 to 14 control write of data and erase of block for the block groups BG₁ to BG₄, respectively.

The control unit 11 includes a write unit 111, a determination unit 112, a selection unit 113, a determination unit 114 and an erase unit 115. The write unit (first write unit) 111 writes data accessed by the processor 2 to the block group BG₁.

The determination unit (first determination unit) 112 determines whether the block group BG₁ satisfies the erase condition (first erase condition) or not.

When the block group BG₁ satisfies the erase condition, the selection unit (first selection unit) 113 selects a block to be erased (first area to be erased) from the block group BG₁.

The determination unit (second determination unit) 114 determines whether each item of data written to the block to be erased is in the first high-use state or the first low-use state and whether each item of the data is data to be deleted, based on the management information 61.

The erase unit (first erase unit) 115 erases the block to be erased when each data item written to the block to be erased can be discarded because each data item is written to the block group BG₂ or BG₃ or data to be deleted.

The control unit 12 includes a write unit 121, a determination unit 122, a selection unit 123, a determination unit 124 and an erase unit 125.

When the determination unit 114 determines that the data written to the block to be erased of the block group BG₁ is in the first low-use state and is not data to be deleted, the write unit (second write unit) 121 writes the data to the block group BG₂.

The determination unit (fifth determination unit) 122 determines whether the block group BG₂ satisfies the erase condition (third erase condition) or not.

When the block group BG₂ satisfies the erase condition, the selection unit (third selection unit) 123 selects a block to be erased (third area to be erased) from the block group BG₂.

The determination unit 124 determines whether each item of data written to the block to be erased is in the third high-use state or the third low-use state and whether each item of the data is data to be deleted, based on the management information 62.

When data that is written to the block to be erased, is in the third high-use state and is not data to be deleted is written to the block group BG₃, the erase unit (second erase unit) 125 erases the data written to the block to be erased.

The control unit 13 includes a write unit 131, a determination unit 132, a selection unit 133, a determination unit 134, a write unit 135, an erase unit 136 and a write unit 137.

When the determination unit 114 determines that data written to the block to be erased of the block group BG₁ is in the first high-use state and is not data to be deleted, the write unit (third write unit) 131 writes the data to the block group BG₃.

When data written to the block group BG₂ is in the third high-use state and is not data to be deleted, the write unit (sixth write unit) 137 writes the data to the block group BG₃. For example, when the data written to the block group BG₂ is data to be accessed by the processor 2, the write unit 137 may write the data to be accessed of the block group BG₂ to the block group BG₃.

The determination unit (third determination unit) 132 determines whether the block group BG₃ satisfies the erase condition (second erase condition) or not.

When the block group BG₃ satisfies the erase condition, the selection unit (second selection unit) 133 selects a block to be erased (second area to be erased) from the block group BG₃.

The determination unit (fourth determination unit) 134 determines whether each item of data written to the block to be erased is in the second high-use state or the second low-use state and whether each item of the data is data to be deleted, based on the management information 63.

When the data written to the block to be erased of the block group BG₃ is determined to be in the second high-use state and be not data to be deleted, the write unit (fifth write unit) 135 writes the data to the other writable block in the block group BG₃ again.

The erase unit (third erase unit) 136 erases the block to be erased when each item of the data written to the block to be erased can be discarded because each data item is written to the block group BG₄, written to the block group BG₃ again, or data to be deleted.

The control unit 14 includes a write unit 141, a determination unit 142, a selection unit 143 and an erase unit 144.

When the determination unit 134 determines that data written to the block to be erased of the block group BG₃ is in the second low-use state and is not data to be deleted, the write unit (fourth write unit) 141 writes the data to the block group BG₄.

The determination unit (sixth determination unit) 142 determines whether the block group BG₄ satisfies the erase condition (fourth erase condition) or not.

When the block group BG₄ satisfies the erase condition (fourth erase condition), the selection unit (fourth selection unit) 143 selects a block to be erased (fourth area to be erased) from the block group BG₄.

The erase unit (fourth erase unit) 144 erases data written to the block to be erased of the block group BG₄.

When data written to the block group BG₂ reaches the third high-use state, the variation unit (first variation unit) 15 increases the number of blocks included in the block group BG₁ and reduces the number of blocks included in the block group BG₃. For example, when the data written to the block group BG₂ is accessed by the processor 2, the variation unit 15 increases the number of blocks included in the block group BG₁ and reduces the number of blocks included in the block group BG₃.

When data written to the block group BG₄ reaches the fourth high-use state, the variation unit (second variation unit) 16 increases the number of blocks included in the block group BG₃ and reduces the number of blocks included in the block group BG₁. For example, when the data written to the block group BG₄ is accessed by the processor 2, the variation unit 16 increases the number of blocks included in the block group BG₃ and reduces the number of blocks included in the block group BG₁.

FIG. 2 is a flowchart showing an example of first cache control according to the present embodiment. FIG. 2 exemplarily shows a process in which data is written to the block group BG₁, the data is written to the block group BG₂ or BG₃ and a block to be erased in the block group BG₁ is erased.

In step S201, the write unit 111 writes data accessed by the processor 2 to the block group BG₁.

In step 202, the determination unit 112 determines whether the block group BG₁ satisfies the erase condition or not.

When the block group BG₁ does not satisfy the erase condition, the process proceeds to step S206.

When the block group BG₁ satisfies the erase condition, the selection unit 113 selects a block to be erased from the block group BG₁ in step S203.

In step S204, the determination unit 114 determines whether each item of data written to the block to be erased is in the first high-use state or the first low-use state and whether each item of the data is data to be erased (deletion target data) based on the management information 61.

When the data item is in the first low-use state and the data is not data to be deleted (non-deletion target data), the write unit 121 writes the data item to the block group BG₂ in step S301.

When the data item is in the first high-use state and the data is not data to be deleted, the write unit 131 writes the data item to the block group BG₃ in step S401.

In step S205, the erase unit 115 erases the block to be erased when each item of the data written to the block to be erased can be discarded because each item of the data is written to the block group BG₂ or block group BG₃, or data to be deleted.

In step S206, the cache control unit 9 determines whether or not to end the process.

When the cache control unit 9 does not end the process, the process returns to step S201.

When the cache control unit 9 ends the process, the process is ended.

FIG. 3 is a flowchart showing an example of second cache control according to the present embodiment. FIG. 3 exemplarily shows a process in which data is written to the block group BG₂ and a block to be erased in the block group BG₂ is erased.

When the data written to the block to be erased of the block group BG₁ is determined to be in the first low-use state and be not data to be deleted in step S204, the write unit 121 writes the data to the block group BG₂ in step S301.

In step S302, the determination unit 122 determines whether the block group BG₂ satisfies the erase condition or not.

When the block group BG₂ does not satisfy the erase condition, the process proceeds to step S306.

When the block group BG₂ satisfies the erase condition, the selection unit 123 selects a block to be erased from the block group BG₂ in step S303.

In step S304, the determination unit 124 determines whether each item of data written to the block to be erased is in the third high-use state or the third low-use state and whether each item of the data is data to be deleted, based on the management information 62.

When the data item is in the third low-use state or is data to be deleted, the process proceeds to step S305.

When the data item is in the third high-use state and is not data to be deleted, the write unit 137 writes the data item to the block group BG₃ in step S401.

In step S305, the erase unit 125 erases the data written to the block to be erased of the block group BG2.

In step S306, the cache control unit 9 determines whether or not to end the process.

When the cache control unit 9 does not end the process, the process returns to step S301.

When the cache control unit 9 ends the process, the process is ended.

FIG. 4 is a flowchart showing an example of third cache control according to the present embodiment. FIG. 4 exemplarily shows a process from writing data to the block group BG₃ to erasing the data in the block group BG₃.

When the data written to the block to be erased of the block group BG₁ is determined to be in the first high-use state and be not data to be deleted in step S204, the write unit 131 writes the data to the block group BG₃ in step S401. When the data written to the block group BG₂ is determined to be in the third high-use state (for example, the data is accessed by the processor 2) and be not data to be deleted in step S304, the write unit 137 writes the data of the block group BG₂ to the block group BG₃.

In step S402, the determination unit 132 determines whether the block group BG₃ satisfies the erase condition or not.

When the block group BG₃ does not satisfy the erase condition, the process proceeds to step S407.

When the block group BG₃ satisfies the erase condition, the selection unit 133 selects a block to be erased from the block group BG₃ in step S403.

In step S404, the determination unit 134 determines whether each item of data written to the block to be erased is in the second high-use state or the second low-use state and whether each item of the data is data to be deleted, based on the management information 63.

When the data item is in the second low-use state and is not data to be deleted, the write unit 141 writes the data to the block group BG₄ in step S501.

When the data is in the second high-use state and is not data to be deleted, the write unit 135 writes the data written to the block to be erased of the block group BG₃ to the other block in the block group BG₃ again in step S405.

In step S406, the erase unit 136 erases the block to be erased when each item of the data written to the block to be erased can be discarded because each data item is written to the block group BG₄, written to the block group BG₃ again, or data to be deleted.

In step S407, the cache control unit 9 determines whether or not to end the process.

When the cache control unit 9 does not end the process, the process returns to step S401.

When the cache control unit 9 ends the process, the process is ended.

FIG. 5 is a flowchart showing an example of fourth cache control according to the present embodiment. FIG. 5 exemplarily shows a process in which data is written to the block group BG₄ and the data in the block group BG₄ is erased.

When the data written to the block to be erased of the block group BG₃ is determined to be in the second low state and be not data to be deleted in step S404, the write unit 141 writes the data to the block group BG₄ in step S501.

In step S502, the determination unit 142 determines whether the block group BG₄ satisfies the erase condition or not.

When the block group BG₄ does not satisfy the erase condition, the process proceeds to step S505.

When the block group BG₄ satisfies the erase condition, the selection unit 143 selects a block to be erased from the block group BG₄ in step S503.

In step S504, the erase unit 144 erases the data written to the block to be erased in the block group BG₄.

In step S505, the cache control unit 9 determines whether or not to end the process.

When the cache control unit 9 does not end the process, the process returns to step S501.

When the cache control unit 9 ends the process, the process is ended.

In the block group BG₁ of the present embodiment, for example, data is first sequentially written to the block B_(1,1), next sequentially written to the block B_(1,2), and then similarily written to the blocks B_(1,3) to B_(1,K). When a data amount of the blocks B_(1,1) to B_(1,K) included in the block group BG₁ exceeds a predetermined data amount, the block B_(1,1) in which writing is first completed is erased by FIFO and data is sequentially written to the erased block B_(1,1) again. After the writing to the block B_(1,1) is completed, the block B_(1,2) is erased by FIFO. Then, data is sequentially written to the erased block B_(1,2) again. The same control is repeated.

In the block group BG₁, whether the data written to the block to be erased in the block group BG₁ is accessed, for example, for less than the first number of times or at less than the first frequency is determined based on the management information 61. When the data written to the block to be erased in the block group BG₁ is accessed for less than the first number of times or at less than the first frequency, the block group BG₂ is selected as a destination of writing of the data.

In contrast, when the data written to the block to be erased in the block group BG₁ is accessed for the first number of times or more or at the first frequency or more, the block group BG₃ is selected as a destination of writing of the data.

When the data written to the block to be erased in the block group BG₁ is data to be deleted, the data is discarded.

In the block group BG₂ of the present embodiment, the data in the first low-use state from the block group BG₁ is first sequentially written to the block B_(2,1), next sequentially written to the block B_(2,2), and then similarly written to the blocks B_(2,3) to B_(2,L). When a data amount of the blocks B_(2,1) to B_(2,L) included in the block group BG₂ exceeds a predetermined data amount, the block B_(2,1) in which writing is first completed is erased by FIFO and data is sequentially written to the erased block B_(2,1) again. After the writing to the block B_(2,1) is completed, the block B_(2,2) is erased by FIFO. Then, data is sequentially written to the erased block B_(2,2). The same control is repeated.

In the block group BG₂, whether the data written to the block to be erased in the block group BG₂ is accessed, for example, for less than the third number of times or at less than the third frequency is determined based on the management information 62. When the data written to the block to be erased in the block group BG₂ is accessed for less than the third number of times or at less than the third frequency, the data is erased.

In contrast, when the data written to the block to be erased in the block group BG₂ is accessed for the third number of times or more or at the third frequency or more, the block group BG₃ is selected as a destination of writing of the data.

When the data written to the block to be erased in the block group BG₂ is data to be deleted, the data is discarded.

In the block group BG₃ of the present embodiment, the data in the first high-use state from the block group BG₁, the data in the third high-use state from the block group BG₂ or the re-write data from the block group BG₃ is first sequentially written to the block B_(3,1), next sequentially written to the block B_(3,2), and then similarly written to the blocks B_(3,3) to B_(3,M). When a data amount of the blocks B_(3,1) to B_(3,M) included in the block group BG₃ exceeds a predetermined data amount, the block B_(3,1) in which writing is first completed is erased by FIFO and data is sequentially written to the erased block B_(3,1) again. After the writing to the block B_(3,1) is completed, the block B_(3,2) is erased by FIFO. Then, data is sequentially written to the erased block B_(3,2) again. The same control is repeated.

In the block group BG₃, whether the data written to the block to be erased in the block group BG₃ is accessed, for example, for less than the second number of times or at less than the second frequency is determined based on the management information 63. When the data written to the block to be erased in the block group BG₃ is accessed for less than the second number of times or at less than the second frequency, the block group BG₄ is selected as a destination of writing of the data.

In contrast, when the data written to the block to be erased in the block group BG₃ is accessed for the second number of times or more or at the second frequency or more, the data is written to the block group BG₃ again.

When the data written to the block to be erased in the block group BG₃ is data to be deleted, the data is discarded.

In the block group BG₄ of the present embodiment, the data in the second low-use state from the block group BG₃ is first sequentially written to the block B_(4,1), next sequentially written to the block B_(4,2), and then similarly written to the blocks B_(4,3) to B_(4,N). When a data amount of the blocks B_(4,1) to B_(4,N) included in the block group BG₄ exceeds a predetermined data amount, the block B_(4,1) in which writing is first completed is erased by FIFO and data is sequentially written to the erased block B_(4,1) again. After the writing to the block B_(4,1) is completed, the block B_(4,2) is erased by FIFO. Then, data is sequentially written to the erased block B_(4,2). The same control is repeated.

In the present embodiment, the control unit 14 may determine whether data written to a block to be erased of the block group BG₄ is in a fifth high-use state or not. When the data written to the block to be erased of the block group BG₄ is determined to be in the fifth high-use state, the control unit 13 may write the data to a writable destination block of the block group BG₃ in respect of maintaining the data in the nonvolatile cache memory 4. In this case, the processor 2 may reduce a size of the block group BG₁.

In the present embodiment, data is managed based on the four block groups BG₁ to BG₄.

For example, first data (once-accessed data) once accessed by the processor 2 is managed in the block group BG₁.

For example, if second data in the block group BG₁ is accessed twice or more by the processor 2 and pushed out from the block group BG₁ based on FIFO, the second data is moved from the block group BG₁ to the block group BG₃.

It should be noted that the size of the block group BG₁ is larger than the size of the block group BG₃ in the present embodiment.

For example, when third data in the block group BG₁ is pushed out from the block group BG₁ based on FIFO without being accessed by the processor 2, the third data is moved from the block group BG₁ to the block group BG₂.

For example, if fourth data in the block group BG₃ is purged from the block group BG₃ based on FIFO without being accessed by the processor 2, the fourth data is moved from the block group BG₃ to the block group BG₄.

For example, in the block groups BG₂ and BG₄, metadata may be cached instead of caching data. The metadata includes information related to the data. In other words, the metadata is highly abstract and additional data on the data and is attached to the data.

In the present embodiment, for example, when fifth data is stored in the block group BG₁, sixth data in the block group BG₂ may be pushed out based on FIFO.

For example, when seventh data in the block group BG₁ is accessed and pushed out from the block group BG₁ based on FIFO, the seventh data may be moved from the block group BG₁ to the block group BG₃, eighth data in the block group BG₃ may be moved from the block group BG₃ to the block group BG₄ based on FIFO and ninth data in the block group BG₄ may be pushed out from the block group BG₄ based on FIFO.

For example, when tenth data in the block group BG₂ is accessed, the size of the block group BG₁ is increased. If the size of the block group BG₁ is increased, eleventh data in the block group BG₃ is moved to the block group BG₄ based on FIFO.

For example, when twelfth data in the block group BG₄ is accessed and push out from the block group BG₄ based on FIFO, the twelfth data is moved to the block group BG₃ and the size of the block group BG₁ is reduced.

In the above-described present embodiment, a maintaining determination determines whether or not to maintain the data for a unit of block, a transfer writing writes the data of block to be maintained to a destination block, and data written to the nonvolatile cache memory 4 is erased per block.

In the present embodiment, an effective cache capacity can be increased, a hit rate of the nonvolatile cache memory 4 can be raised and a speed of the information processing device 17 can be increased.

In the present embodiment, a decrease in performance can be avoided without performing garbage collection for the nonvolatile cache memory 4. Since garbage collection is not necessary, the number of writes to the nonvolatile cache memory 4 can be reduced and the life of the nonvolatile cache memory 4 can be increased. Furthermore, since garbage collection is not necessary, provisioning areas do not need to be secured. Therefore, a data capacity available as a cache memory can be increased and usage efficiency can be improved.

For example, when the nonvolatile memory is used as a cache memory and data is discarded regardless of the boundaries of blocks, garbage collection may be frequently performed to move valid data in a block of the nonvolatile memory to another block. In the present embodiment, garbage collection does not need to be performed in the nonvolatile cache memory 4. Therefore, as described above, the life of the nonvolatile cache memory 4 can be increased in the present embodiment.

Second Embodiment

The present embodiment is a modified example of the first embodiment. In the present embodiment, transmission and reception of data and information between the information processing device 17 including the cache control unit 9 and the SSD 5 are described.

In the present embodiment, a logical address is used as identification information of data. However, the data may be identified by other information.

FIG. 6 is a block diagram showing a configuration example of the information processing system 35 according to the present embodiment.

In addition to the composition elements described in the first embodiment, the cache control unit 9 further includes a transmission unit 18, a reception unit 19, a write unit 20 and a transmission unit 21.

The transmission unit 18 transmits, to the SSD 5, write data for the SSD 5 and an address of the write data. In the present embodiment, the address transmitted from the transmission unit 18 to the SSD 5 is, for example, a logical address.

The reception unit 19 receives, from the SSD 5, block information including logical addresses indicative of valid data written to a block to be subjected to garbage collection.

In the present embodiment, the block information may include information relating identification information of each block in the SSD 5 to identification information of data written to each block.

The write unit 20 writes (transcribes) all or a part of the valid data indicated by the logical addresses included in the block information to a memory other than the nonvolatile memory 24 based on the block information received from the SSD 5 and the management information 61 to 64. The other memory may be, for example, the nonvolatile cache memory 4.

For example, the write unit 20 excludes a logical address indicative of data which is data to be deleted(deletion candidate) in the case of receiving a delete command, from the logical addresses indicative of the valid data included in the block information. Thus, the valid data that is written to the block to be subjected to garbage collection and is not data to be deleted can be selected. The write unit 20 writes the selected data to the other memory.

The transmission unit 21 generates deletion information including the logical address indicative of the data is to be deleted and transmits the deletion information to the SSD 5. For example, the deletion information may include a logical address indicative of data which is the deletion target that is not written to the other memory by the write unit 20, of the logical addresses indicative of the valid data included in the block information. Instead of the deletion information, maintaining information including logical addresses of data to be maintained may be transmitted from the transmission unit 21 to the SSD 5.

The SSD 5 includes a processor 22, a memory 23 and the nonvolatile memory 24.

For example, the memory 23 stores various types of control data such as address translation information 32, valid/invalid information 33 and deletion information 34. The memory 23 may be a volatile memory such as a DRAM or an SRAM or may be a nonvolatile memory. The memory 23 may be included in the nonvolatile memory 24.

The processor 22 functions as an address translation unit 25, a write unit 26, a valid/invalid generation unit 27, a selection unit 28, a transmission unit 29, a reception unit 30 and a garbage collection unit 31 by executing a program stored in a memory in the processor 22, a program stored in the memory 23 or a program stored in the nonvolatile memory 24.

In the present embodiment, the program to cause the processor 22 to function as the address translation unit 25, the write unit 26, the valid/invalid generation unit 27, the selection unit 28, the transmission unit 29, the reception unit 30 and the garbage collection unit 31 may be, for example, the OS, middleware or firmware. In the present embodiment, all or a part of the address translation unit 25, the write unit 26, valid/invalid generation unit 27, the selection unit 28, transmission unit 29, the reception unit 30 and the garbage collection unit 31 may be implemented by hardware.

When the write data and the logical address of the write data is received from the cache control unit 9, the address translation unit 25 generates information relating the logical address of the write data to a physical address indicative of a position in the nonvolatile memory 24 in which the write data is stored, and registers the information to the address translation information 32.

In the present embodiment, the address translation unit 25 is implemented by the processor 22. However, the address translation unit 25 may be configured separately from the processor 22.

The address translation unit 25 translates addresses based on, for example, the table-form address translation information 32. Instead, addresses may be translated by key-value retrieval. For example, address translation can be implemented by means of key-value retrieval by using a logical address as a key and a physical address as a value.

The write unit 26 writes the write data to the position indicated by the physical address obtained by the address translation unit 25.

The valid/invalid generation unit 27 generates valid/invalid information 33 indicating whether each item of the data written to the nonvolatile memory 24 is valid data or invalid data based on, for example, the address translation information 32. Then, the valid/invalid generation unit 27 stores the valid/invalid information 33 in the memory 23.

The selection unit 28 selects a block to be subjected to garbage collection.

For example, the selection unit 28 may select a block having the oldest write time from blocks in the nonvolatile memory 24 as a block to be subjected to garbage collection.

For example, the selection unit 28 may select a block to be subjected to garbage collection at random from the blocks in the nonvolatile memory 24.

For example, the selection unit 28 may select a block having the largest amount of invalid data or having the amount of invalid data larger than a predetermined amount as a block to be subjected to garbage collection, based on the valid/invalid information 33.

For example, the selection unit 28 may select a block having the largest amount of invalid data and data to be deleted or having the amount of invalid data and data to be deleted larger than a predetermined amount as a block to be subjected to garbage collection, based on the valid/invalid information 33 and the deletion information 34.

The transmission unit 29 generates block information by deleting a logical address indicative of invalid data determined as being invalid by the valid/invalid information 33 from logical addresses indicative of data written to the block to be subjected to garbage collection. In other words, the block information includes information relating identification information of the block to be subjected to garbage collection to logical addresses indicative of valid data written to the block. The transmission unit 29 transmits the block information to the cache memory control unit 9.

The reception unit 30 receives the deletion information from the cache memory control unit 9 and stores the deletion information 34 in the nonvolatile memory 24.

The garbage collection unit 31 excludes invalid data and data to be deleted from the data written to the block to be subjected to garbage collection based on the valid/invalid information 33 and the deletion information 34 stored in the nonvolatile memory 24, and executes garbage collection only for valid data that is not data to be deleted.

FIG. 7 is a flowchart showing an example of a process performed by the information processing system according the present embodiment.

In step S701, the transmission unit 18 transmits write data and a logical address to the SSD 5.

In step S702, the address translation unit 25 receives the write data and the logical address and registers, to the address translation information 32, information relating the logical address of the write data to a physical address.

In step S703, the write unit 26 writes the write data to a position in the nonvolatile memory 24 indicated by the physical address.

In step S704, the valid/invalid generation unit 27 generates valid/invalid information 33 indicating whether each item of data written to the nonvolatile memory 24 is valid data or invalid data, and stores the valid/invalid information 33 in the memory 23.

In step S705, the selection unit 28 selects a block to be subjected to garbage collection.

In step S706, the transmission unit 29 generates block information by deleting a logical address indicative of invalid data indicated as being invalid by the valid/invalid information 33 from logical addresses indicative of data written to the block to be subjected to garbage collection, and transmits the block information to the cache control unit 9.

In step S707, the reception unit 19 receives the block information from the SSD 5.

In step S708, the write unit 20 writes all or a part of data indicated by the logical addresses included in the block information to a memory other than the nonvolatile memory 24 of the SSD 5, based on the block information received from the SSD 5 and the management information 61 to 64.

For example, the write unit 20 excludes a logical address indicative of data to be deleted in the case of receiving a delete command from the logical addresses included in the block information, and writes data indicated by the logical addresses to be maintained to the other memory.

In step S709, the transmission unit 21 transmits the deletion information including the logical address of data to be deleted to the SSD 5.

In step S710, the reception unit 30 receives the deletion information from the cache control unit 9 and stores the deletion information 34 in the memory 23.

In step S711, the garbage collection unit 31 excludes invalid data and data to be deleted data from the data written to the block to be subjected to garbage collection based on the valid/invalid information 33 and the deletion information 34, and executes garbage collection for valid data that is not data to be deleted.

In the above-described present embodiment, the cache control unit 9 can acquire information on data written to a block of the nonvolatile memory 24 from the SSD 5. The cache control unit 9 can thereby recognize a state of write of the data in the block of the nonvolatile memory 24. For example, in the present embodiment, whether the data written to the block of the nonvolatile memory 24 is valid data or invalid data and whether the data may be deleted can be recognized.

In the present embodiment, the SSD 5 includes the valid/invalid information 33 to determine whether the data is valid data or invalid data and the deletion information 34 to determine whether the data may be deleted or not. Whether or not to erase data written to a block to be subjected to garbage collection can be thereby determined when garbage collection is performed in the SSD 5. Therefore, an unnecessary write of data can be avoided and the life of the nonvolatile memory 24 can be increased.

In the present embodiment, the cache control unit 9 can prevent the deletion target data amongst the valid data indicated by the logical addresses included in the block information received from the SSD 5 from being transcribed from the nonvolatile memory 24 to the other memory. In the present embodiment, the SSD 5 can delete data that is not transcribed from the cache control unit 9 to the other memory (for example, invalid data or valid data that may be deleted) from the SSD 5.

In the above-described present embodiment, the block information related to the block to be erased is transmitted from the SSD 5 to the information processing device 17. However, the block information may include, for example, information relating each block in the nonvolatile memory 24 to identification information of data written to each block. The information processing device 17 can recognize the relationship of storage between the blocks and the data in the SSD 5 by receiving the relation information from the SSD 5.

Third Embodiment

In the present embodiment, the information processing system 35 including the information processing system 17 and SSD 5 explained in the first and second embodiments are further explained in detail.

FIG. 8 is a block diagram showing of an example of a detail structure of the information processing system 35 according to the present embodiment.

The information processing system 35 includes the information processing device 17 and a memory system 37.

The SSD 5 according to the first and second embodiments corresponds to the memory system 37.

The processor 22 of the SSD5 corresponds to a CPU 43B.

The address translation information 32 corresponds to an LUT (Look Up Table) 45.

The memory 23 corresponds to a DRAM 47.

The information processing device 17 functions as a host device.

A controller 36 of the memory system 37 includes a front end 4F and a back end 4B.

The front end (host communication unit) 4F includes a host interface 41, host interface controller 42, encode/decode unit (Advanced Encryption Standard (AFS)) 44, and CPU 43F.

The host interface 41 communicates with the information processing device 17 to exchange requests (write command, read command, erase command), LBA(Logical Block Addressing), and data.

The host interface controller (control unit) 42 controls the communication of the host interface 41 based on the control of the CPU 43F.

The encode/decode unit 44 encodes the write data (plaintext) transmitted from the host interface controller 42 in a data write operation. The encode/decode unit 44 decodes encoded read data transmitted from the read buffer RB of the back end 4B in a data read operation. Note that the transmission of the write data and read data can be performed without using the encode/decode unit 44 as occasion demands.

The CPU 43F controls the above components 41, 42, and 44 of the front end 4F to control the whole function of the front end 4F.

The back end (memory communication unit) 4B includes a write buffer WB, read buffer RB, LUT 45, DDRC 46, DRAM 47, DMAC 48, ECC 49, randomizer RZ, NANDC 50, and CPU 43B.

The write buffer (write data transfer unit) WB stores the write data transmitted from the information processing device 17 temporarily. Specifically, the write buffer WB temporarily stores the data until it reaches to a predetermined data size suitable for the nonvolatile memory 24.

The read buffer (read data transfer unit) RB stores the read data read from the nonvolatile memory 24 temporarily. Specifically, the read buffer RB rearranges the read data to be the order suitable for the information processing device 17 (the order of the logical address LBA designated by the information processing device 17).

The LUT 45 is a data to translate the logical address an LBA into a physical address PBA(Physical Block Addressing).

The DDRC 46 controls double data rate (DDR) in the DRAM 47.

The DRAM 47 is a nonvolatile memory which stores, for example, the LUT 45.

The direct memory access controller (DMAC) 48 transfers the write data and the read data through an internal bus IB. In FIG. 8, only a single DMAC 48 is shown; however, the controller 36 may include two or more DMACs 48. The DMAC 48 may be set in various positions inside the controller 36.

The ECC (error correction unit) 49 adds an error correction code (ECC) to the write data transmitted from the write buffer WB. When the read data is transmitted to the read buffer RB, the ECC 49, if necessary, corrects the read data read from the nonvolatile memory 24 using the added FCC.

The randomizer RZ (or scrambler) disperses the write data in such a manner that the write data are not biased in a certain page or in a word line direction of the nonvolatile memory 24 in the data write operation. By dispersing the write data in this manner, the number of write can be standardized and the cell life of the memory cell MC of the nonvolatile memory 24 can be prolonged. Therefore, the reliability of the nonvolatile memory 24 can be improved. Furthermore, the read data read from the nonvolatile memory 24 passes through the randomizer RZ in the data read operation.

The NAND controller (NANDC) 50 uses a plurality of channels (four channels CHO to CH3 are shown in the Figure) to access the nonvolatile memory 24 in parallel in order to satisfy a demand for a certain speed.

The CPU 43B controls each component above (45 to 50, and RZ) of the back end 4B to control the whole function of the back end 4B.

Note that the structure of the controller 36 is an example and no limitation is intended thereby.

FIG. 9 is a perspective view showing an example of a storage system according to the present embodiment.

A storage system 100 includes the memory system 37 as an SSD.

The memory system 37 is, for example, a relatively small module of which external size will be approximately 20 mm×30 mm. Note that the size and scale of the memory system 37 is not limited thereto and may be changed into various sizes arbitrarily. Furthermore, the memory system 37 may be applicable to the information processing device 17 as a server used in a data center or a cloud computing system employed in a company (enterprise) or the like. Thus, the memory system 37 may be an enterprise SSD (eSSD).

The memory system 37 includes a plurality of connectors (for example, slots) 38 opening upwardly, for example. Each connector 38 is a serial attached SCSI (SAS) connector or the like. With the SAS connector, a high speed mutual communication can be established between the information processing device 17 and each memory system 37 via a dual port of 6 Gbps. Note that, the connector 38 may be a PCI express (PCIe) or NVM express (NVMe).

A plurality of memory systems 37 are individually attached to the connectors 38 of the information processing device 17 and supported in such an arrangement that they stand in an approximately vertical direction. Using this structure, a plurality of memory systems 37 can be mounted collectively in a compact size, and the memory systems 37 can be miniaturized. Furthermore, the shape of each memory system 37 of the present embodiment is 2.5 inch small form factor (SFF). With this shape, the memory system 37 can be compatible with an enterprise HDD (eHDD) and the easy system compatibility with the eHDD can be achieved.

Note that the memory system 37 is not limited to the use in an enterprise HDD. For example, the memory system 37 can be used as a memory medium of a consumer electronic device such as a notebook portable computer or a tablet terminal.

As can be understood from the above, the information processing system 35 and the storage system 100 having the structure of the present embodiment can achieve a mass storage advantage with the same advantages of the second embodiment.

The structure of the memory system 37 according to the present embodiment may be applied to the information processing device 17 according to the first embodiment. For example, the processor 2 according to the first embodiment may correspond to the CPU43B. The address translation information 7 may correspond to the LUT 45. The memory 3 corresponds to the DRAM 47. The nonvolatile cache memory 4 may correspond to the nonvolatile memory 24.

While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions. 

What is claimed is:
 1. A cache memory device comprising: a nonvolatile cache memory comprising a first group including a plurality of first erase unit areas, a second group including a plurality of second erase unit areas and a third group including a plurality of third erase unit areas, each of the first to third erase unit areas including a plurality of write unit areas; a first write unit which writes data to the first group; a generation unit which generates management information indicative of a state of use of the data written to the first group; a first determination unit which determines whether the first group satisfies a first erase condition or not; a first selection unit which selects a first area to be erased from the first group when the first group satisfies the first erase condition; a second determination unit which determines, based on the management information, whether the data written to the first area to be erased is in a first high-use state or a first low-use state; a second write unit which writes the data to the second group when the data is determined to be in the first low-use state; a third write unit which writes the data to the third group when the data is determined to be in the first high-use state; and a first erase unit which erases the data written to the first area to be erased.
 2. The cache memory device according to claim 1, wherein the nonvolatile cache memory further comprises a fourth group including a plurality of fourth erase unit areas, and the cache memory device further comprises: a third determination unit which determines whether the third group satisfies a second erase condition or not; a second selection unit which selects a second area to be erased from the third group when the third group satisfies the second erase condition; a fourth determination unit which determines, based on the management information, whether the data written to the second area to be erased is in a second high-use state or a second low-use state; a fourth write unit which writes the data written to the second area to be erased to the fourth group when the data written to the second area to be erased is determined to be in the second low-use state; a fifth write unit which writes the data written to the second area to be erased to the third group again when the data written to the second area to be erased is determined to be in the second high-use state; and a second erase unit which erases the data written to the second area to be erased.
 3. The cache memory device according to claim 1, wherein the first determination unit determines that the first erase condition is satisfied when a data amount of the first group exceeds a predetermined amount.
 4. The cache memory device according to claim 1, wherein the first selection unit selects the first area to be erased from the first group based on first-in first-out.
 5. The cache memory device according to claim 2, further comprising: a first variation unit which increases the number of the first erase unit areas included in the first group and reduces the number of the third erase unit areas included in the third group when the data written to the second group reaches a third high-use state; and a second variation unit which increases the number of the third erase unit areas included in the third group and reduces the number of the first erase unit areas included in the first group when the data written to the fourth group reaches a fourth high-use state.
 6. The cache memory device according to claim 5, further comprising: a sixth write unit which writes the data written to the second group to the third group when the data written to the second group reaches the third high-use state.
 7. The cache memory device according to claim 2, further comprising: a fifth determination unit which determines whether the second group satisfies a third erase condition or not; a third selection unit which selects a third area to be erased from the second group when the second group satisfies the third erase condition; a third erase unit which erases the data written to the third area to be erased; a sixth determination unit which determines whether the fourth group satisfies a fourth erase condition or not; a fourth selection unit which selects a fourth area to be erased from the fourth group when the fourth group satisfies the fourth erase condition; and a fourth erase unit which erases the data written to the fourth area to be erased.
 8. The cache memory device according to claim 7, wherein the third selection unit selects, based on first write times or first write orders of the plurality of second erase unit areas, an erase unit area indicating that the first write time or the first write order is old, as the third area to be erased, and the fourth selection unit selects, based on second write times or second write orders of the plurality of fourth erase unit areas, an erase unit area indicating that the second write time or the second write order is old, as the fourth area to be erased.
 9. A cache memory device comprising: a nonvolatile cache memory including a plurality of erase unit areas, each of the erase unit areas including a plurality of write unit areas; a write unit which writes data to the nonvolatile cache memory; a determination unit which determines whether the plurality of erase unit areas satisfy an erase condition or not; a selection unit which selects an area to be erased from the plurality of erase unit areas when the plurality of erase unit areas satisfy the erase condition; and an erase unit which erases the data written to the area to be erased.
 10. The cache memory device according to claim 9, wherein the erase condition is a data amount of the plurality of erase unit areas exceeding a predetermined amount.
 11. The cache memory device according to claim 9, wherein the area to be erased is an area of an erase unit size, and the erase unit erases all the data written to the area to be erased.
 12. The cache memory device according to claim 9, further comprising: a generation unit which generates management information indicative of a state of use of the data written to the plurality of erase unit areas; a maintaining determination unit which determines, based on the management information, whether or not to maintain the data written to the area to be erased in the nonvolatile cache memory; and a transfer write unit which writes the data written to the area to be erased to a destination erase unit area included in the nonvolatile cache memory when the data written to the area to be erased is determined to be maintained in the nonvolatile cache memory.
 13. The cache memory device according to claim 12, wherein the management information includes identification information of the data, information indicating whether the data is data to be deleted or not, and state-of-use information of the data.
 14. The cache memory device according to claim 9, wherein the selection unit selects, based on write times or write orders of the plurality of erase unit areas, an erase unit area indicating that the write time or the write order is old from the plurality of erase unit areas as the area to be erased.
 15. The cache memory device according to claim 9, wherein the selection unit selects the area to be erased from the plurality of erase unit areas at random.
 16. A non-transitory computer readable recording medium storing a program causing a computer to function as: a write unit which writes data to the nonvolatile cache memory, the nonvolatile cache memory including a plurality of erase unit areas, each of the erase unit areas including a plurality of write unit areas; a determination unit which determines whether the plurality of erase unit areas satisfy an erase condition or not; a selection unit which selects an area to be erased from the plurality of erase unit areas when the plurality of erase unit areas satisfy the erase condition; and an erase unit which erases the data written to the area to be erased. 